A participating medium is one that absorbs, scatters, or emits light and thereby “participates” in light transport into or through it. Examples of participating media include smoke, clouds, and dust, as well as marble statuary, flesh, and milk or murky water. Due to their rich organic structure, participating media are widely used in modern computer-generated graphics. For example, participating media are used to convey natural depth and scale cues and can provide meaningful shot composition.
Despite their value to computer graphics, however, incorporating participating media into light transport simulations presents a substantial computational challenge. For instance, producing an adequate representation of participating media within a conventional Monte Carlo path tracing framework can introduce prohibitive computational complexity.